This invention relates to a method and reactor for manufacturing objects by solid freeform fabrication, especially titanium and titanium alloy objects. Structured metal parts made of titanium or titanium alloys are conventionally made by casting, forging or machining from a billet. These techniques have a disadvantage of high material use of the expensive titanium metal and large lead times in the fabrication.
Fully dense physical objects may be made by a manufacturing technology known as rapid prototyping, rapid manufacturing, layered manufacturing or additive fabrication. This technique employs computer aided design software (CAD) to first construct a virtual model of the object which is to be made, and then transform the virtual model into thin parallel slices or layers, usually horizontally oriented. The physical object may then be made by laying down successive layers of raw material in the form of liquid paste, powder or sheet material resembling the shape of the virtual layers until the entire object is formed. The layers are fused together to form a solid dense object. In case of depositing solid materials which are fused or welded together, the technique is also termed as solid freeform fabrication.
Solid freeform fabrication is a flexible technique allowing creation of objects of almost any shape at relatively fast production rates, typically varying from some hours to several days for each object. The technique is thus suited for formation of prototypes and small production series, but less suited for large volume production.
The technique of layered manufacturing may be expanded to include deposition of pieces of the construction material, that is, each structural layer of the virtual model of the object is divided into a set of pieces which when laid side by side form the layer. This allows forming metallic objects by welding a wire onto a substrate in successive stripes forming each layer according to the virtual layered model of the object, and repeating the process for each layer until the entire physical object is formed. The accuracy of the welding technique is usually too coarse to allow directly forming the object with acceptable dimensions. The formed object will thus usually be considered a green object or pre-form which needs to be machined to acceptable dimensional accuracy.
Taminger and Hafley [1] disclose a method and device for manufacturing structural metal parts directly from computer aided design data combined with electron beam freeform fabrication (EBF). The structural part is built by welding on successive layers of a metallic welding wire which is welded by the heat energy provided by the electron beam. The process is schematically shown in FIG. 1, which is a facsimile of FIG. 1 of [1]. The EBF process involves feeding a metal wire into a molten pool made and sustained by a focused electron beam in a high vacuum environment. The positioning of the electron beam and welding wire is obtained by having the electron beam gun and the positioning system (the support substrate) movably hinged along one or more axis (X, Y, Z, and rotation) and regulate the position of the electron beam gun and the support substrate by a four axis motion control system. The process is claimed to be nearly 100% efficient in material use and 95% effective in power consumption. The method may be employed both for bulk metal deposition and finer detailed depositions, and the method is claimed to obtain significant effect on lead time reduction and lower material and machining costs as compared to the conventional approach of machining the metal parts. The electron beam technology has a disadvantage of being dependent upon a high vacuum of 10−1 Pa or less in the deposition chamber.
It is known to use a plasma arc to provide the heat for welding metallic materials. This method may be employed at atmospheric or higher pressures, and thus allow simpler and less costly process equipment. One such method is known as gas tungsten arc welding (GTAW, also denoted as TIG) where a plasma transferred arc is formed between a non-consumable tungsten electrode and the welding area. The plasma arc is usually protected by a gas being fed through the plasma torch forming a protective cover around the arc. TIG welding may include feeding a metal wire or metal powder into the melting pool or the plasma arc as a filler material.
From U.S. Patent Application Publication No.: 2010/0193480 it is known to employ a TIG-welding torch to build objects by solid free-form fabrication (SFFF), where successive layers of metallic feedstock material with low ductility are applied onto a substrate. A plasma stream is created by energizing a flowing gas using an arc electrode, the arc electrode having a variable magnitude current supplied thereto. The plasma stream is directed to a predetermined targeted region to preheat the predetermined targeted region prior to deposition. The current is adjusted and the feedstock material is introduced into the plasma stream to deposit molten feedstock in the predetermined targeted region. The current is adjusted and the molten feedstock is slowly cooled at an elevated temperature, typically above the brittle to ductile transition temperature of the feedstock material, in a cooling phase to minimize the occurrence of material stresses.
Another example is U.S. Patent Application Publication No.: 2006/0185473 which discloses use of TIG torch in place of the expensive laser traditionally used in a solid freeform fabrication (SFFF) process with relatively low cost titanium feed material by combining the titanium feed and alloying components in a way that considerably reduces the cost of the raw materials. More particularly, in one aspect the present invention employs pure titanium wire (CP Ti) which is lower in cost than alloyed wire, and combines the CP Ti wire with powdered alloying components in-situ in the SFFF process by combining the CP Ti wire and the powder alloying components in the melt of the welding torch or other high power energy beam. In another embodiment, the invention employs titanium sponge material mixed with alloying elements and formed into a wire where it may be used in an SFFF process in combination with a plasma welding torch or other high power energy beam to produce near net shaped titanium components.
Titanium metal or titanium alloys heated above 400° C. may be subject to oxidation upon contact with oxygen. It is thus necessary to protect the weld and heated object which is being formed by layered manufacture against oxygen in the ambient atmosphere.
One solution to this problem is known from International Patent Application Publication No.: WO 2011/0198287 which discloses a method for increasing the deposition rate by performing the manufacturing of objects by solid freeform fabrication, especially titanium and titanium alloy objects, in a reactor chamber which is closed to the ambient atmosphere. By making the deposition chamber sufficiently void of oxygen, the need for employing protective measures to avoid oxidizing the newly welded area by ambient atmospheric oxygen is no longer present, such that the welding process may proceed at a larger velocity since the welded zone may be allowed to have a higher temperature without risking excessive oxidation of the weld. For example, in production of objects of titanium or titanium alloy, there is no longer need for cooling the welded zone to below 400° C. to avoid oxidation.
Another solution for increasing the deposition rate is known from U.S. Pat. No. 6,268,584 which discloses a deposition head assembly consisting of the following features: an array of output powder nozzles for creating a converging flow of powder to the deposition region, a central orifice which allows the multiple beams to be focused onto the deposition substrate, and coaxial gas flow for each of the powder nozzles to concentrate the stream of powders from these nozzles in order to provide a longer working distance between the nozzle and the deposition head assembly. The longer working distance is critical to insure that molten metal particulates are not attached to the deposition apparatus during processing. In particular, the invention includes a manifold system designed into the deposition head assembly that can use more than one laser beam simultaneously for the deposition process. The deposition head assembly also incorporates a means for actively concentrating the powder stream from each orifice to increase material utilization efficiency.
International Patent Application Publication No.: WO 2006/0133034 discloses use of combined gas metal arc and laser welding to solve the problems associated with the reactive nature of Ti and its molten characteristics which make it very difficult to form DMD products. Gas metal arc techniques have several disadvantages that severely limit their application to depositing Ti. These drawbacks include instabilities in metal transfer, excessive spatter, and poor control of the deposited layer shape, and high heat input that causes distortion of thin sections during deposition. Also, an increase in productivity is not possible because of wandering of the cathode spot that occurs during deposition. The solution to these problems according to International Patent Application Publication No.: WO 2006/133034 is to a direct metal deposition process comprising the steps of providing a substrate and depositing a metal from a metal feedstock onto the substrate. An electric arc is generated between the metal feedstock and the substrate and the arc is exposed to laser radiation to form a molten metal pool on the substrate. The molten metal pool is cooled to form a first solid metal layer on the substrate.